Transport in Vascular Plants

http://bcs.whfreeman.com/thelifewire/content/chp00/00020.html

Chpt 35 : REVIEW SECONDARY GROWTH

Chpt 36: TRANSPORT IN PLANTS

http://glencoe.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/

Phloem loadingWater intakeOsmosisEtc.

Text Summary of Transport

http://www.wou.edu/~bledsoek/103materials/chapter_notes/103Ch42b.pdf

Vascular land plants = plant body of roots (absorb H2O & minerals) & shoots (absorb light and CO2)

Transport sometimes over long distances

Xylem transports H2O & minerals from roots to shoots

Phloem transports sugars, etc. from where synth (source) to where needed (sink)

I. Physical forces drive the transport of materials in vascular plants

A. 3 levels of transport

1. Transport of water and solutes by individual cells, e.g., root hairs via plasmodesmata

2. Short-distance transport of substances from cell to cell at the levels of tissues and organs, e.g. loading sugar from photosynthetic leaf cells sieve tubes of phloem

3. Long-distance transport within xylem & phloem throughout whole plant

Minerals

H2O O2

O2

CO2 O2

H2OSugar

Light

A variety of physical processes are involved in the different levels of transport

Sugars are produced byphotosynthesis in the leaves.

5

Sugars are transported asphloem sap to roots and otherparts of the plant.

6

Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.

4

Transpiration, the loss of waterfrom leaves (mostly through

stomata), creates a force withinleaves that pulls xylem sap upward.

3

Water and minerals aretransported upward from

roots to shoots as xylem sap.

2

Roots absorb waterand dissolved minerals

from the soil.

1

Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.

7

CO2

1. Roots absorb H2O & minerals from soil2. Transported up to shoots as xylem sap

Minerals

H2O

H2O

3. Transpiration (loss H2O) through stomata creates force in leaves that pulls xylem sap up

Minerals

H2O

CO2 O2

H2O

4. Through stomata, leaves take in CO2 & expel O2 (CO2 for photosynth, O2 from cell resp)

Minerals

H2O

CO2 O2

H2O Sugar

Light

5. Sugar made by photosynth in leaves

6. Sugar transported as phloem sap to roots & other parts where needed

Minerals

H2O CO2

O2

CO2 O2

H2O Sugar

Light

7. Roots exchange gases with air spaces of soil (O2 used in catabolism of sugars)

B. Selective Permeability of Membranes

• Selective permeability of plant cell’s plasma membrane controls movement of solutes into & out of cell

• Specific transport proteins enable plant cells to maintain internal environment different from their surroundings

Review: passive and active transport compared

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. No energy is required.

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.

ATP

Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,

Diffusion. Hydrophobicmolecules and (at a slow rate) very small uncharged polar molecules can diffuse through the lipid bilayer.

Gated channel

Most solutes cannot cx memb without help of transport proteins

B. Central Role of Proton Pumps Most impt active transport prot in plants is

the proton pump in plant cells

CYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+ H+

H+

H+

H+H+

H+

Proton pump generates membrane potentialand H+ gradient.

–

––

–

– +

+

+

+

+

- Creates a hydrogen ion gradient (a form of potential energy that can be harnessed to do work)- Contributes to a voltage known as a membrane potential (also potential energy)

Plants use energy stored in proton gradient & memb potential to drive transport of different solutes

+CYTOPLASM EXTRACELLULAR FLUID

Cations ( , for example) are driven into the cell by themembrane potential.

Transport protein

K+

K+

K+

K+

K+ K+

K+

K+

–

–

– +

+

1. Uptake of K+

–

–

+

+

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

NO3–

NO 3 –

NO3

–

NO 3

–

NO3

–

NO3 –

–

–

– +

+

+

–

–

– +

+

+

NO3–

2. Cotransport couples downhill passage of (H+) with concommitant uphill passage of (NO3) this = active transport

H+of through a

cotransporter.

Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion

H+

H+

H+

H+

H+H+

H+

H+ H+

H+

SS

S

S

SPlant cells canalso accumulate a neutral solute,such as sucrose

( ), bycotransporting

down the

steep protongradient.

S

H+

–

–

–

+

+

+

–

–

++–

H+ H+S+

–

3. Uptake of sucrose with contransport of H+ moving down its conc gradient in uptake of sucrose by plant cells

C. Water Potential

To survive plants must balance water uptake & loss

Osmosis (diffusion of H2O acx selectively permeable memb)

responsible for net uptake or loss of water

• Water potential (Ψ) is a measurement that combines effects of solute conc & pressure. to determine the direction of H2O movement

• Water flows from regions of high water potential to regions of low water potential

• Solute contribution to water potential of a solution is proportional to the number of dissolved molecules

• Pressure contribution to water potential of a solution is the physical pressure on a solution (involves plant cell wall)

• Addition of solutes reduces Ψ

0.1 Msolution

H2O

Purewater

P = 0

S = 0.23

= 0.23 MPa = 0 MPa

(a)

• Application of physical pressure increases Ψ

H2O

P = 0.23

S = 0.23

= 0 MPa = 0 MPa

(b)

H2O

P = 0.30

S = 0.23

= 0.07 MPa = 0 MPa

(c)

• Negative pressure decreases Ψ

H2O

P = 0

S = 0.23

= 0.23 MPa

(d)

P = 0.30

S = 0

= 0.30 MPa

• If a flaccid cell is placed in an environment with a higher solute conc (hypertonic soln), the cell will lose water & plasmolyze (memb will shrink away from its cell wall)Flaccid = limp. A walled cell is flaccid in surroundings where there is no tendency for water to enter

Hypotonic solutionHypertonic solution

• If the same flaccid cell is placed in a soln with a solute concentration lower than that in the protoplast, the cell will gain water and become turgid (very firm) as cell wall pushes back against enlarging memb. A walled cell becomes turgid if it has a greater solute conc than its surroundings, resulting in entry of water.

• Loss of turgor (due to loss of water in environment) in plants causes wilting which can be reversed when the plant is watered.

Healthy plants are turgid most of the time.

D. Aquaporin and Water Transport

● Aquaporins = transport prots in memb that allow the passage of water

● Do not affect water potential

Plasmodesma

Plasma membrane

Cell wallCytosol

Vacuole

Ke

Symplast

Apoplast

Vacuolar membrane(tonoplast)

E. 3 Major Compartments Vacuolated Cells

Transport also regulated by compartmental structure of plant cells

1. cell wall (maintain cell shape) edge of space

2. plasma membrane (controls H2O in/out) & edge of protoplast (contents of cell less wall)

3. vacuole

• Plasma membrane

- Directly controls the traffic of molecules in/out protoplast

- Is a barrier between two major compartments, cell wall & cytosol

• 3rd major compartment vacuole = a large organelle that can occupy as much as 90% of more of the protoplast’s volume

• Vacuolar membrane regulates transport between the cytosol and the vacuole

Transport proteins inthe plasma membrane

regulate traffic ofmolecules betweenthe cytosol and the

cell wall.

Transport proteins inthe vacuolarmembrane regulatetraffic of moleculesbetween the cytosoland the vacuole.

PlasmodesmaVacuolar membrane

Plasma membrane

Cell wall

Cytosol

Vacuole

Cell compartments. The cell wall, cytosol, and vacuole are the three maincompartments of most mature plant cells.

Key

Symplast

Apoplast

ApoplastTransmembrane route

Symplastic route

Symplast

1. transmembrane route (out of memb-ax cell wall-into another cell)

2. symplastic route (cell memb to cell directly vis dermatoplasmata)

3. apoplastic route (stay outside cells)

F. Short lateral transport via one of 3 ways:Symplast = continuous cytosol, cell to cell via plasmodesmata Apoplast = continuum cell walls & extracellular spaces

Substances may transfer from one route to another.

Apoplastic route

These 3 ways from root hairs to vascular cylinder

Water & minerals travel short distances from root hairs to vascular cylinder of root via 3 lateral (not

up/down) routes

1. Transmemb: out of one cell, across a cell wall, & into another cell = repeated crossing plasma memb

2. Via symplast == continum of cytosol; only one crossing of plasma membrane & then cell-to-cell via plasmodermata

3. Along apoplast == continuem of cell wall; no entering protoplast

Can change from one route to another route

G. Bulk Flow for Long-Distance Transport is movement of fluid in the xylem & phloem

driven by pressure differences at opposite ends of the xylem vessels and sieve tubes

Diffusion OK for short distances, but too slow for long distances …. need bulk flow

Water & fluids move through tracheids & vessels of xylem & sieve tubes of phloem

Tracheids = tapered cells with pitsVessel elements = wider, long channels, end walls perforated for easy flow

DEADALIVE

Sieve tube member = cell with sieve plate

Companion Cell = nonconducting cntd via plasmodesmata

Phloem: loading of sugar = high + pressure at opposite end of sieve tube = mvt fluid

Xylem: negative pressure tension by transpiration from leaves pulls sap up from roots

Cytoplasm of sieve-tube members almost devoid of internal organelles & porous sieve plates = easier flow

Dead tracheids & vessel elements (porous end walls) have no cytoplasm to inhibit flow

Bulk flow due to pressure differences = way long-distance transport of phloem sap & active transport of sugar at cellular level maintains pressure difference

II. Roots absorb water & minerals from soil

enter the plant through the epidermis of roots, cx root cortex, pass into vascular cylinder & ultimately bulk flow to shoot system

Root hairs account for much surface area of roots

• Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil

• Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with

fungal hyphae = increase surface area of roots2.5 mm

Lateral transport of minerals and water in roots

1

2

3

Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls.

Minerals and water that crossthe plasma membranes of roothairs enter the symplast.

As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove inward via the symplast.

Within the transverse & radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks thepassage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder.

Endodermal cells & also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water & minerals upward into shoot system.

Pathwaythroughsymplast

Plasmamembrane

Casparian strip

Apoplasticroute

Symplasticroute

Root hair

Epidermis Cortex EndodermisVascular cylinder

Vessels(xylem)

Casparian strip

Endodermal cell

4 5

2

1

Root hairs (extensions of epidermal cells) absorb

Passes freely apoplastic route into cortex Cells of epidermis & cortex take up = into symplast

Endodermis: innermost layer cells in root cortex, surrounds vascular cylinder & is last checkpoint for selective passage of minerals from cortex into vascular tissue

Minerals/water from roots into symplast of epidermis or cortex, continue via plasmodesmata of endodermal cells into vascular cylinder

Minerals/water from roots into apoplast meet Casparian strip (dead end & cannot enter vascular cylinder via apoplast) BUT can enter symplast and enter

Waxy belt in walls endodermal cells; impervious to water/minerals

• III. Water and minerals ascend from roots to shoots through the xylem

• Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant

• The transpired water must be replaced by water transported up from the roots

Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism is the major pressure driving flow of xylem sap up to shoot system

• Water is pulled upward by negative pressure in the xylem produced by transpiration through stomata

• Cohesion of water (H bonding) transmits upward pull along length of sylem to roots

IV. Stomata help regulate rate of transpiration

Leaves generally have broad surface areas & high surface-to-volume ratios that increase photosynthesis & increase water loss

● Plants lose a large amount of water by transpiration

● If lost water is not replaced by absorption through roots, plant will lose water & wilt

● Transpiration = evaporative cooling which lowers T of leaf to prevent denaturation of various enzs involved in photosynthesis & other metabolic processes

Each stoma is flanked by guard cells that control diameter by changing shape

V. Nutrients translocated through phloem

Phloem sap (an aqueous solution, mostly sucrose) that translocated from source to sink

A sugar source : a plant organ that is a net producer of sugar, such as mature leaves

A sugar sink: an organ that is a net consumer or storer of sugar, such as a tuber or bulb

Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members.

In angiosperms sap moves through a sieve tube by bulk flow driven by positive pressure

Phloem loading may be via active transport; proton pumping & cotransport of sucrose & H+

1. Load sucrose (green) into sieve tube at source = ↓Ψ in sieve tube = causes sieve tube to take up water by osmosis

2. Uptake water = ↑ ΨP to force sap to flow

3. At sink, sugar unloaded = ↓ ΨP

& water out via osmosis

Phloem sap flows from source to sink via bulk flow mediated by pressure